Such rotary hammers are known in numerous forms (European Patent No. 0 014 760, U.S. Pat. No. 4,280,359, U.S. Pat. No. 4,750,567). In all these rotary hammers, a piston is moved to and fro driven coaxial to the rotation axis of a bit located in a tool holder of the rotary hammer in order to reciprocate a ram coaxial to the piston via the alternating development of overpressure and underpressure, by which ram the desired impacts are created on the rear end of the bit. The front side of the ram is connected to the surrounding air via at least one vent opening in order that, upon forward movement of the ram, air located in the space in front of the latter can be expelled without noteworthy resistance, so that the ram can strike with all of its available energy either the rear end of the bit or an anvil arranged between the rear end of the bit and the ram. This vent opening also ensures that, upon rearward movement of the ram, sufficient air can be sucked into the zone at the front side of the ram that no underpressure arises there which would reduce the rearward movement of the ram and thus the impact energy available during operation.
It is also already known (European Patent No. 0 358 978) to change the impact energy of a rotary hammer by changing the stroke of the driven reciprocatable piston. However, this requires a relatively complicated mechanism.
In U.S. Pat. No. 5,111,890 a rotary hammer is shown which comprises a tool body, a tubular casing in the tool body, a piston reciprocable in the casing and a ram reciprocable in the casing and adapted to impact a hammer bit, means for developing an alternating over and under pressure behind the ram to cause the ram to reciprocate and impact the bit and vent holes in the tubular casing which vent the air in front of the ram to the atmosphere. Associated with these vent holes are throttle holes, and the arrangement is such that, when the hammer is in an idle condition, the vent holes in front of the ram are closed, and the throttle holes are open, thus allowing for the creation of a slight overpressure in front of the ram to prevent the ram fully impacting the bit. When the hammer is in an operative, hammering condition, the vent holes in front of the ram are open and the throttle holes are closed, thus allowing the ram to impact the bit fully. Thus the arrangement does not allow for any variation of the force of impact of the ram on the bit when the hammer is in its operative, hammering, condition.
The object of the invention is to provide an improved rotary hammer in which a reduction in the impact energy transmitted from the ram onto the bit is easily effected.